Dont allow guest to set assigne, milestone and label when create new issue or merge request

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/ability.rb

@@ -154,6 +154,9 @@ class Ability
       project_guest_rules + [
         :download_code,
         :fork_project,
+        :set_assignee,
+        :set_milestone,
+        :set_label,
         :write_project_snippet
       ]
     end

app/views/shared/issuable/_form.html.haml

@@ -37,8 +37,9 @@
 
       .clearfix
       .error-alert
-%hr
-.form-group
+  %hr
+- if can?(current_user, :set_assignee, @project)
+  .form-group
     .issue-assignee
       = f.label :assignee_id, class: 'control-label' do
         %i.fa.fa-user
@@ -49,7 +50,8 @@
             selected: issuable.assignee_id, project: @target_project || @project)
          
         = link_to 'Assign to me', '#', class: 'btn assign-to-me-link'
-.form-group
+- if can?(current_user, :set_milestone, @project)
+  .form-group
     .issue-milestone
       = f.label :milestone_id, class: 'control-label' do
         %i.fa.fa-clock-o
@@ -64,7 +66,8 @@
          
         - if can? current_user, :admin_milestone, issuable.project
           = link_to 'Create new milestone', new_namespace_project_milestone_path(issuable.project.namespace, issuable.project), target: :blank
-.form-group
+- if can?(current_user, :set_label, @project)
+  .form-group
     = f.label :label_ids, class: 'control-label' do
       %i.fa.fa-tag
       Labels